The present invention pertains to transparent thermoplastic polyamides obtained from a cycloaliphatic diamine, a lactam, or the corresponding amino acid containing at least 7 carbon atoms; and terephthalic acid or a mixture of isophthalic and terephthalic acids containing more than 50% terephthalic acid.
Transparent polyamides prepared from a mixture of isophthalic and terephthalic acids containing more than 50 mol. % terephthalic acid are described in U.S. Pat. No. 3,847,877. These products are obtained by polycondensation of the mixture of acids, of bis(4-aminocyclohexyl) methane and specifically of epsilon-caprolactam. The defects of the resulting product are that it has a significant capacity to absorb water and becomes cloudy when treated in boiling water.
Transparent polyamides prepared from lactam containing at least 7 carbon atoms, or from the corresponding omega-amino acid, are also known from French Patent No. 2,324,672. These polyamides are prepared from in addition to lactam, cycloaliphatic diamine and a mixture of isophthalic and terephthalic acids.
However, the polyamides described in this patent contain a maximum of only 50 mol. % terephthalic acid in the mixture of terephthalic and isophthalic acids, and, because of this, have very little resistance to gasoline containing alcohol. According to this patent, the polyamides are obtained by polycondensation in a single stage and under pressure when the reaction is conducted in the presence of lactam. This process has the disadvantage of producing polyamides of mediocre transparency, or even opaque polyamides when they are prepared from a mixture of acids containing more than 50 mol. % terephthalic acid.